This invention relates to a method of producing printing plates. More specifically, the invention relates to a method of producing printing plates having microdots in non-image areas.
In the past various methods have been employed to prevent stains on the background, i.e. to prevent ink attached to the non-image area from staining non-printed portions of a material to be printed.
One method is to use a printing plate having a relatively high relief. Typographic printing plates of lead alloy base, or a photosensitive resin or etched metallic plates are com. As a means of preventing staining the background the relief was relatively high, e.g. 0.5 mm or higher relief.
Printing plates having relatively high reliefs have several disadvantages including difficulty in handling and fastening to a rotary printing cylinder; use of a relatively large quantity of materials with resultant high production costs; and lengthy production time. In many ways printing plates with a low relief are preferable.
A second method for preventing staining the background is to provide a printing plate with microdots. Originally, such microdots had the same height as that of the printing relief and were arranged on the total surface of a printing face of the printing plate. Therefore, what was printed using such printing plate became wholly dark. Further, in the case of printing with a colored ink, small colored spots caused by the microdots were conspicuous. The resultant printed matter was unacceptable.
As an improvement in the method for using microdots in the past it was also suggested that the height of microdots should not exceed that of the printing relief. This second method was superior to the first in that if printing working conditions were appropriate, the material was not printed with microdots. Even where such conditions were bad, the ink was only attached to a non-printed area by virtue of microdots.
Use of a transparent or semi-transparent support was also suggested. In such a method two image carriers were used. A layer of photocurable polymer composition was placed on the top of a transparent or semi-transparent backing sheet and actinic radiation was applied to the polymer composition from one side through the microdot image carrier and from the other side through the printing image carrier respectively, thereby to produce a printing plate having microdots.
This method which required two different image carriers was restricted largely by the fact that the backing sheet had to be made of a transparent or semi-transparent material. Generally, a transparent or semi-transparent plastic material such as polyethylene terephthalate, polystyrene, polycarbonate, polyvinyl chloride, polypropylene, polyvinyl alcohol, etc. was used; however, sheets formed from these materials are not always dimensionally stable. Therefore, in cases where accuracy or precision were critical this was a fatal problem. Further, when considering the heat generated by the actinic radiation, the dimensional instability was not negligible even for general printing.
Further, in the prior method of using a transparent or semi-transparent backing sheet it was necessary to bring the microdot image carrier into as close contact with a photosensitive resin layer as possible. Because of this, the above method has various problems in view of working efficiency, etc.
Moreover, with the method of making use of the above transparent or semi-transparent backing sheet there is the problem of fastening the printing plate to the printing cylinder. That is, as the suitable method of fastening the printing plate to a printing cylinder, for example, a magnetic clamping system is often used. In this method, the printing plate is attached to the rotary printing cylinder by means of a magnetic force. However, in the above described method it was necessary for the backing sheet to be transparent or semi-transparent. A transparent or semi-transparent ferromagnetic substance is not usually appropriate. Therefore, the above magnetic clamp cannot be used.
Additionally, in the method utilizing the above transparent or semi-transparent backing sheet there is a problem of temperature control of the photocurable polymer composition. To maintain exact thickness of the layer of the polymer composition or to cure such composition adequately, the temperature of the polymer composition on the top of the backing sheet must be controlled accurately. It is necessary in the method utilizing the above described transparent or semi-transparent backing sheet to apply radiation from both sides. This makes it impossible to have a temperature control device in close contact with the backing sheet. Therefore accurately controlling the temperature is difficult.
The present invention was discovered and improves the above situation. The object of the present invention is to provide a new method of producing a printing plate with microdots.
Another object of the invention is to provide a method for producing a printing plate wherein the material of the backing sheet is not restricted, thereby enabling one to use a dimensionally stable backing sheet.
A further object is to provide a method for producing a printing plate with microdots wherein it is not necessary to bring the microdot image carrier into close contact with the layer of a polymer composition, thereby providing excellent working efficiency.
Another object is to provide a method for producing a printing plate with microdots in which the backing sheet can be formed with a ferromagnetic material, so that the backing sheet can be fastened to a rotary printing cylinder using a magnetic clamp system.
A further object is to provide a method for the production of a printing plate with microdots in which accurate temperature control of photosensitive resins can be performed easily.
Another object is to provide a method for producing printing plates with only one irradiation of actinic radiation wherein said printing plate has a printing relief with adequate height and microdots with a height lower than that of the printing relief.
For a better understanding of the present invention, together with other and further objects, reference is made to the following description, taken in conjunction with the accompanying drawings and its scope will be pointed out in the appended claims.